Writing good horror involves more than creating gross scenes you hope will shock your readers. The truth is, your audience is jaded, and it is getting more jaded all the time. Writing a horror story that will truly scare your readers requires you to reach their fears on a deep level. This means writing about the things you know will scare them, and to get to that point, you need to start by asking yourself what scares you.
There are three ways you can make your horror story stand out. First, take a universal fear, or something you know frightens a lot of people, and find a way to exploit that fear to make your readers uncomfortable. If you can make your readers feel uncomfortable, you're on the right track. A good horror story disturbs readers. If something you write disturbs even a small portion of your readers, if it keeps a few of them up at night, then you are doing your job.
The second thing you can do to give your horror story a boost is to write about real-life horror. If your readers recognize reality mixed with horror, they are likely to be afraid. If they read something that could conceivably happen to them, you will have found a way to scare them.
The third thing that can help make your horror a cut above the rest is to write the unexpected. You've read the horror books and seen the scary movies. You know what the expected frights are. Some are traditional and almost necessary in the genre, but look for other ways to accomplish the same effect. Change the way you write a scare. When it's expected, change course and wait. Bring it in later, when your audience isn't expecting it. It's a challenge to scare a jaded horror audience. Take the challenge.
Combine other genres with your horror to give the stuff you write a new twist. Western horror, erotic horror, science fiction horror, and romantic horror are cool combinations that allow you to combine the elements of horror with elements found in other genres. This can make for a more interesting story.
Remember to follow the same rules of good storytelling in horror as you do in all genres. Characters, plot structure, and a story that makes sense within the world it's written. These are important to any type of writing. Make them important to your horror fiction.